Ballistic articles such as bulletproof vests, helmets, structural members of helicopters and other military equipment, vehicle panels, briefcases, raincoats and umbrellas containing high strength fibers are known. Fibers conventionally used include aramids such as poly(phenylenediamine terephthalamide), graphite fibers and the like. For many applications, such as vests or parts of vests, the fibers are used in a woven or knitted fabric. For many of the other applications, the fiber is encapsulated or embedded in a composite material.
A number of properties are generally considered to be necessary for the high strength fiber to be useful as a ballistic resistant material. Four of these factors listed by John V. E. Hansen and Roy C. Laible, in "Fiber Frontiers" ACS Conference, (June 10-12, 1974) entitled "Flexible Body Armor Materials" are higher modulus, higher melting point, higher strength and/or work-to-rupture values and higher resistance to cutting or shearing. With regard to melting point, it is indicated as desirable to retard, delay or inhibit the melting seen with nylon and polyester. In a book entitled "Ballistic Materials and Penetration Mechanics", by Roy C. Laible (1980), it is indicated that no successful treatment has been developed to bring the ballistic resistance of polypropylene up to the levels predicted from the yarn stress-strain properties (page 81) and that melting in the case of nylon and polyester fibers may limit their ballistic effectiveness. Laible indicated that NOMEX, a heat resistant polyamide fiber with modest strength, possesses fairly good ballistic resistant properties (page 88).
Furthermore, in "The Application of High Modulus Fibers to Ballistic Protection" R. C. Laible et al., J. Macromol. Sci.-Chem. A7(1), pp. 295-322 1973, it is indicated on p. 298 that a fourth requirement is that the textile material have a high degree of heat resistance; for example, a polyamide material with a melting point of 255.degree. C. appears to possess better impact properties ballistically than does a polyolefin fiber with equivalent tensile properties but a lower melting point. In an NTIS publication, AD-A018 958 "New Materials in Construction for Improved Helmets", A. L. Alesi et al., a multilayer highly oriented polypropylene film material (without matrix) referred to as "XP" was evaluated against an aramid fiber (with a phenolic/polyvinyl butyral resin matrix). The aramid system was judged to have the most promising combination of superior performance and a minimum of problems for combat helmet development.